a. Field of the Invention
The present invention relates generally to composite material structures, and, more particularly, to an attachment fitting at which generally axial loads are transmitted into such a composite material structure.
b. Related Art
Composite materials are used in the manufacture of a wide variety of structures, especially where a high strength-to-weight ratio is desired. As is known to those skilled in the art, and as is used in this description, composite materials (or simply xe2x80x9ccompositesxe2x80x9d) are materials in which fibers (usually formed of a high tensile strength material) are imbedded in a resin matrix. Well known examples of composite materials include glass fiber-resin composites and graphite fiber-resin composites, the latter being particularly noted for high strength and light weight. Such materials are available from a large number of manufacturers, one example being Hexcel Corporation of Pleasanton, Calif., USA.
In recent years, composite materials have found increasing use in comparatively large, high-load applications, notably in air frames and orbital satellites, where minimization of weight is a critical consideration. In these applications, the composite material structures are ordinarily bolted or otherwise mounted in load-bearing attachment to one or more other components or structures, such as to struts, panels, rods, bars, brackets and so on, and an attachment fitting of some type is included in the composite material structure for this purpose. In order to provide a precise, durable attachment point, such fittings are ordinarily made of metal, with a socket or pin for attachment of the other member.
Since the attachment fitting is normally formed of metal, a common problem arises as to how to mount the fitting to the fiber-resin material of the composite structure. In some instances, an adhesive has been employed for this purpose, but to form a strong bond requires expensive types of adhesives which are difficult and time-consuming to work with, and the adhesive joint almost invariably represents a weak spot. In other instances, flanges may be formed on the inner and outer ends of the fitting to engage the surfaces of the composite material structure; this results in an excessively heavy fitting, which negates the purpose of using a composite material in the first place, and moreover the transfer/distribution of the loads into the composite material structure is less than ideal.
Accordingly, there exists a need for an attachment fitting for composite material structures which is light in weight and which can be mounted to such structures without forming a weak point. Furthermore, there exists a need for such an attachment fitting which can be efficiently mounted to such a structure without requiring adhesives or the like to form the joint between the two. Still further, there exists a need for such a fitting which will transfer generally axial loads into the composite material structure in an efficient and evenly distributed manner, so as to obtain the full benefit of the load carrying capacity offered by the composite material. Still further, there exists a need for a method for mounting such an attachment fitting to a composite material structure in an efficient and inexpensive manner.
The present invention has solved the problems cited above, and is a load-bearing attachment fitting for use in composite material structures, and a method for use of the same.
The attachment fitting is an elongate member having a series of generally sinusoidal undulations along its longitudinal sides, and a mounting portion attaching the fitting to an external structure. The ridges and grooves of the sinusoidal surface are configured for load-bearing engagement of a surrounding fiber-resin matrix.
The elongate member of the attachment fitting may be a generally cylindrical member, and the sinusoidal undulations may form a series of generally annular ridges and grooves along the cylindrical member. The means for mounting the fitting to an adjacent structure may be a generally axial bore through the fitting, and the bore may have internal threads over at least a portion thereof.
The composite material structure may be an elongate strut having first and second attachment fittings mounted in axial alignment at opposite ends of the strut.
In accordance with the method of the present invention, a composite material structure is fabricated by mounting the attachment fitting adjacent an internal form member and laying at least one fiber-resin layer over the fitting and form member so that the fitting and form member are joined thereby, and so that at least a portion of the fibers in the fiber-resin layer engage the sinusoidal ridges and grooves of the attachment fitting.
The at least one fiber-resin layer may be at least one lengthwise fiber layer in which the fibers thereof extend in a direction generally parallel to generally axial loads which are applied to the attachment fitting. The at least one lengthwise fiber-resin layer may be overlain with at least one crosswise fiber layer in which the fibers extend generally circumferentially around the attachment fitting so as to retain the lengthwise fibers therein.
The lengthwise fiber-resin layer may comprise at least one strip of fiber-resin tape which is placed over the fitting and form, and the crosswise fiber-resin layer may comprise fiber which is wound over the fitting and the lengthwise fibers.
The form may comprise a fluidizable plaster mandrel. After casting, the plaster mandrel may be removed by washing this out through the threaded bore in the attachment fitting. dr
FIG. 1 is an elevational view of an attachment fitting in accordance with the present invention, showing the generally sinusoidal contour of the longitudinal sides thereof;
FIG. 2 is an end view of the attachment fitting of FIG. 1, showing the generally cylindrical configuration of the fitting, and the flats along the sides thereof which prevent the fitting from turning within the composite material body;
FIG. 3 is an elevational view of a longitudinal cross-section taken through the attachment fitting of FIG. 1, showing the internally threaded bore thereof;
FIG. 4 is an elevational view of a second attachment fitting which is generally similar to that which is shown in FIG. 1, but which is somewhat longer for increased load bearing capacity;
FIGS. 5-13 are a series of views showing the sequential steps in the formation of a fiber-resin composite structure having an attachment fitting in accordance with the present invention, FIG. 5 being an elevational view of a generally cylindrical mandrel over which an exemplary composite material strut is formed in accordance with the method of the present invention;
FIG. 6 is an elevational view, similar to FIG. 5, showing conical mandrel pieces having been placed at the ends of the main cylindrical mandrel, and also showing the attachment fittings of the present invention having been placed against the outer ends of the conical end pieces and secured in place by removable collars;
FIG. 7 is an elevational view of the assembly of FIG. 6, showing the manner in which a first fiber layer is formed by helical winding of fiber material thereon;
FIG. 8 is an elevational view, partly in cross-section, showing the formation of one or more axially aligned fiber layers by placement of a series of longitudinal fiber-resin strips over the assembly;
FIG. 9 is an elevational view showing the formation of an outer fiber layer in which the fiber material is again wound onto the assembly, with the helically-wound fibers serving to hold the axial fibers in conformity with the sinusoidal surfaces of the attachment fittings at the ends of the assembly;
FIG. 10 is an elevational view of the assembly of FIG. 11, showing an outer layer of removable tape being wound thereon for retaining the fiber layers in position during subsequent heating and curing of the materials;
FIG. 11 is a cross-sectional view showing the exemplary composite material strut which has been constructed in FIGS. 5-10 being cured at an elevated temperature in an autoclave;
FIG. 12 is an elevational view of the assembly of FIG. 10, showing the outer tape layer being removed after the fiber-resin layers have been cured to form a rigid strut member;
FIG. 13 is an elevational view of the strut member of FIG. 11, showing the removal of the internal mandrel by washing this out with a flow of water or other fluid; and
FIG. 14 is a partial, cross-sectional view of an end portion of the finished strut of FIG. 13, showing the manner in which the series of cured fiber-resin layers engage the sinusoidal exterior of the attachment fitting so as to retain the fitting in the composite material structure and effect the transfer of loads thereto.